In a wireless communication system, a base station communicates with a plurality of remote terminals, such as cellular mobile telephones. Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) are the conventional multiple access schemes for delivering services simultaneously to a certain number of terminals. The basic idea underlying FDMA and TDMA systems is to respectively share the available resource in several frequencies or several time slots in such a way that several terminals can operate simultaneously without causing interference.
Telephones operating according to the GSM standard belong to FDMA and TDMA systems in the sense that transmission and reception are carried out at different frequencies and also using different time slots. In contrast to these systems using a division of frequency or a division of time, Code Division Multiple Access (CDMA) systems allow several users to share a common frequency and a common time channel using a coded modulation. Among the CDMA systems, there is the CDMA 2000 system, the WCDMA (Wideband CDMA) system or the IS-95 standard.
In CDMA systems, as is well known to those skilled in the art, a scrambling code is associated with each base station and enables one base station to be differentiated from another. Furthermore, an orthogonal code, known as an OVSF code, is allocated to each remote terminal, for example, a cellular mobile telephone. All of the OVSF codes are orthogonal with respect to each other, and this enables one remote terminal to be differentiated from another. Before transmitting a signal over the transmission channel intended for a remote terminal, the signal has been scrambled and spread by the base station using the scrambling code of the base station and the OVSF code of the remote terminal.
In CDMA systems, a further distinction can be made between those systems which use a separate frequency for transmission and reception (CDMA-FDD system), and those which use a common frequency for transmission and reception but separate time domains for transmission and reception (CDMA-TDD system).
In UMTS systems, a receiver normally referred to as a Rake receiver is used. This receiver is used to carry out time alignment, descrambling, and despreading for combining delayed versions of the signal initially transmitted by the base station. These versions result from the multi-path transmission characteristics of the transmission medium. In particular, each path introduces a different delay.
The Rake receiver conventionally has a certain number of fingers respectively corresponding to the different paths of the transmission medium. The Rake receiver receives at its input the values of the different delays of each path, and also the value of the fading coefficients of the transmission channel paths. The fading coefficients of the paths of the multi-path transmission channel are to be determined. Since these coefficients are complex coefficients, the amplitude and the phase of the fading of each path are determined. This determination forms part of the channel estimation.
In the context of channel estimation, which is necessary for any coherent reception, as is the case for the UMTS standard, each time slot includes a transmission frame and for each finger of the Rake receiver, that is for each path, the phase and amplitude of the fading are estimated.
Among the known approaches for estimating these parameters, there is the method which includes a straightforward correlation of the received signal with the product of the spreading and scrambling code. However, such a method is very sub-optimal since it does not use the observations arising from the signal.
Among the optimal methods, there is the Wiener method which is well known to the person skilled in the art. The Wiener method involves filtering the estimations in the sense of maximum likelihood by a Wiener filter. This filter is constructed from the inversion of the correlation matrix of the signal. Moreover, this method assumes knowledge of the bits transmitted. This method, although optimal in terms of performance, remains extremely complex to implement.